Input: add zinitix touchscreen driver

Input: add zinitix touchscreen driver

Add support for the bt541 touchscreen IC from zinitix, loosely based on
downstream driver. The driver currently supports multitouch (5 touch points).
The bt541 seems to support touch keys, but the support was not added because
that functionality is not being utilized by the touchscreen used for testing.
Based on the similartities between downstream drivers, it seems likely that
other similar touchscreen ICs can be supported with this driver in the future.

Signed-off-by: Michael Srba <Michael.Srba@seznam.cz>
Link: https://lore.kernel.org/r/20201001122949.16846-1-michael.srba@seznam.cz
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>